No, it’s not about maths being the source of truth. It’s about variety in your game experience.
There is nothing wrong in someone spending 100-1000s of hours playing with the very limited number of the “best” picks from the perspective of “meta”, but there is plenty of us who consider those picks boring and purposefully avoid “hyper-optimised” set-ups. Living by the “meta” for me would literally cut out content from the game. There is plenty of new experiences to have with other, not-so-optimal setups and weapons.
That variety, personal preferences (I like “enter-an-obscure-weapon-name-here”) and seeking a self-imposed extra difficulty modifier provides a lot of people with new content you purposefully ignore, because “my dps meter and other metric say that X is the best setup”.
People are motivated by different things. In your own words: I find it strange that I have to clarify this 
Also if you consider “maths” or “meta” the only metric which picks your weapons and builds for you, it doesn’t matter what Fatshark would have done to those old weapons. With your current approach, someone would “crunch” the numbers, and proclaim this and that is the current best setup according to the numbers and again lock you in a narrow set of meta picks.
There is no way Fatshark could make all builds and weapons equally attractive with such strict expectations. I mean, in the end of the day, something will always have lower DPS, armour pen, mobility or whatever else metric you bring up in an attempt to find the new “best” set-up.
Also, I’m not saying go play with a grey weapon or something. I’m saying pick a weapon from the pile, consider its characteristics and make a build around its strengths and weaknesses, even if weapon X, Y or Z on that class would be better on paper.
To not be too confrontational, I agree there is plenty of ranged weapons in Darktide which feel meh, aren’t fun to use for me. To the point I would never use them, but then I don’t play Tide games for great shooter experience. Some weapons just feel like relics from other time, when Darktide was a different game.